1. Field of Invention
This invention relates to magnetic memory devices and, more particularly, to recovery of data when there is an error during a read operation.
2. Description of Prior Art
A magnetic memory device, such as a disk drive or a tape drive, typically has a magnetic head or transducer and a magnetic medium formatted into multiple data tracks. The magnetic medium is moved past a station where the transducer is located. The transducer is positioned to write or read data to or from addressable locations contained along the tracks.
The magnetic memory device also includes a read/write signal processing section that processes a read signal produced by the transducer before handing it over to a computer or other device that requested the data. The signal processing section includes a preamplifier for amplifying the read signal.
The signal to noise ratio (SNR) is a significant determinant of the read back performance of the magnetic memory device. For a given memory capacity design, a minimum SNR is usually required to meet the device""s soft error rate target. A high amplitude output signal from the head can saturate the preamplifier to produce a distorted signal that increases noise, thereby resulting in a degraded SNR. If the amplitude of output signal from the head is too low for the voltage gain amplifier in the channel:, the SNR can also be degraded. Both of these conditions can lead to higher bit error rate. It is known that, due to bit crowding, the amplitude of the read signal varies with track location. For example, when the head is positioned above an inner track on a magnetic disk medium, the amplitude is low. When it is positioned above an outer track, the amplitude is high.
U.S. Pat. No. 4,772,964 deals with this situation by varying the gain of the preamplifier according to track position. A computer provides a gain value for each position of the transducer to the preamplifier. The gain value selects the value of a resistor that determines gain of the preamplifier. Thus, the gain is reduced for a high amplitude signal and is increased for a low amplitude signal. The gain is also adjusted based on the ambient temperature in which the magnetic memory device is situated. The temperature measurement is not needed when the ambient temperature is controlled to a constant temperature.
U.S. Pat. No. 5,519,548 discloses a procedure that uses read channel noise to calibrate amplifier gain and discrimination thresholds in the read/write signal processing circuitry of a magnetic memory device. The calibration procedure achieves a final threshold value that ensures that the read signal must exceed a predetermined signal to noise ratio before the read signal is accepted as valid. The calibration procedure is performed before searching for a head alignment reference signal on the magnetic medium.
Neither of the aforementioned patents addresses recovery of errors caused by variations in spacing or gap between the head and the disk surface due to debris, asperity of the disk surface or due to temperature and pressure change in the hard disk drive.
It is an object of the present invention to provide an apparatus that recovers the data of a read signal that is in error due to variations in head to disk gap caused by disk surface asperity or hard disk drive temperature/pressure change and to thermal asperity events.
It is another object of the present invention to provide a method of recovering the data of a read signal that is in error due to variations in head to disk gap caused by disk surface asperity or hard disk drive temperature/pressure change and to thermal asperity events.
It is another object of the present invention to provide a memory medium that stores a program that controls the signal processing section of a read/write channel to recover the data of a read signal that is in error due to variations in head to disk gap caused by disk surface asperity or hard disk drive temperature/pressure change and to thermal asperity events.
A method for recovering data according to the present invention is operative during a read operation of a magnetic memory device when an error is detected. The magnetic memory device has a moving magnetic medium upon which data is stored at addressable locations, a transducer for reading the data to produce a read signal and an adjustable gain amplifier for amplifying the read signal. The transducer is spaced from the moving medium by a normal gap distance.
The data recovery method of the present invention involves producing a first read signal by reading a first data from one of the addressable locations. Next, the first read signal is examined for an error. If an error is detected, the method tests for a variation from the normal gap distance. If a variation is found, the amplifier gain is adjusted dependent upon the variation. Next, the first data is reread to produce a second read signal. It is then determined if the second read signal is error free. If the second read signal is error free, the first data is recovered from the second read signal and provided to an output of the magnetic memory device.
The variation from the normal gap distance can be caused by an asperity of the magnetic medium surface, such as an accumulation of debris or a roughness or unevenness. In such case, the variation is a decrease from the normal gap distance that results in a higher amplitude read signal that saturates the amplifier. For this situation, the amplifier gain is decreased.
The variation from the normal gap distance can be caused by a change in disk temperature from a reference temperature, such as the medium temperature at the time of manufacture, and the moving medium. If the temperature increases, the gap decreases and the amplifier gain is decreased. If the temperature decreases, the gap increases and the amplifier gain is increased.
The data recovery apparatus and memory medium of the present invention involve the procedure described above for the data recovery method.